Republican presidential candidate Donald Trump speaks during a campaign rally at the Phoenix Convention Center, Aug. 31, 2016, in Phoenix, Arizona. After considering a softer position, Trump has now reverted to his original hard stance on immigration.

U.S. Republican presidential candidate Donald Trump has ended his flirtation with softer immigration policies, declaring he would deport millions of undocumented immigrants living in the country and renewing his vow to build a wall on its southern border and make Mexico pay for it.

"There will be no amnesty!" leading to citizenship for immigrants already in the U.S., Trump wrote on Twitter Thursday. "Mexico will pay for the wall -- 100%."

Trump laid out his tough immigration plans if he wins the November election against Democrat Hillary Clinton in a speech late Wednesday in the southwestern border state of Arizona, long one of the focal points of U.S. efforts to stem the stream of illegal immigration into the U.S. from Mexico and Central America. Hours earlier, Trump made a quick visit to Mexico City for talks on immigration and other issues with Mexican President Enrique Pena Nieto.

U.S. officials believe there are about 11 million undocumented immigrants living in the U.S. and Trump's plans could lead to the deportation of more than six million of them, as many as two million who have committed crimes and another 4.5 million who have overstayed their visas to enter the U.S. Trump left unclear the fate of other immigrants who entered the country without documentation but have not committed crimes.

"There will be no legal status or becoming a citizen of the United States by illegally entering our country," Trump told cheering supporters.

FILE - A U.S. Border Patrol agent drives near the U.S.-Mexico border fence in Sunland Park, New Mexico, Jan. 4, 2016. Trump pledges to replace the fence with a wall that Mexico will pay for.

Upholding ‘laws of the nation’

On Thursday, speaking at the annual convention of a veterans' group, the American Legion, Trump said, "We are going to uphold the laws of the nation and defend our sovereignty and security and we are going to defend our border."

Trump said that he and Mexico's Pena Nieto "can work together to accomplish great things for both countries," but their differences remain over Trump's border wall plan.

Trump, a real estate mogul making his first run for elected office, said after the Mexico City meeting that he and Pena Nieto discussed the plan, but not Trump's assertion that Mexico will cover the cost. Pena Nieto, however, later said he made clear Mexico would not pay for it.

"They don't know it yet, but they're going to pay for the wall," Trump told the Arizona rally. The crowd enthusiastically chanted, "Build that wall!"

Democratic candidate Hillary Clinton's campaign chairman John Podesta said Trump "choked" by not fully addressing the issue with Pena Nieto.

"What we saw today from a man who claims to be the ultimate 'deal maker' is that he doesn't have the courage to advocate for his campaign promises when he's not in front of a friendly crowd," Podesta said.

‘No one will be immune’

After weeks of reports that Trump might be softening his immigration platform, he proposed hiring 5,000 new border patrol agents, tripling the number of immigration enforcement officers and immediately deporting anyone caught illegally entering the country. He said crime and border crossings would plummet and gangs would disappear.

"People will know that you can't just smuggle in, hunker down and wait to be legalized," he told the crowd in Arizona. "No one will be immune or exempt from enforcement."

Demonstrators are arrested outside the White House in Washington, Aug. 28, 2014, during a protest for immigration reform. Trump's plans could lead to the deportation of more than six million illegal immigrants.

Trump called for the completion of a biometric entry and exit system to better track those who enter the country, and for the expansion of e-verify systems to help employers avoid hiring illegal immigrants.

He also reiterated plans to suspend the issuing of visas to those living in countries where people cannot be adequately screened to ensure they are not a security risk. Those admitted to the U.S. would have to pass ideological tests as well.

"We also have to be honest about the fact that not everyone who seeks to join our country will be able to successfully assimilate," he said.

Clinton also wants to continue current U.S. policy to deport those who have committed crimes, but Trump objects to executive orders signed by President Barack Obama offering deferred enforcement to certain categories of people with no criminal record.

Clinton wants to protect them, which Trump has labeled as "amnesty." Clinton, a former U.S. secretary of state, has also called for a comprehensive overhaul of U.S. immigration policies similar to legislation that has stalled with Republican opposition in the House of Representatives after being approved by the Senate.